Viruses & Prions

Viruses

&

Prions

• Virus – miniscule, acellular, infectious agent

having one or several pieces of either DNA

or RNA

• No cytoplasmic membrane, cytosol,

organelles

• Cannot carry out any metabolic pathway

• Neither grow nor respond to the environment

• Cannot reproduce independently

• Obligate intracellular parasites

Viruses

Viruses

• Viruses contain DNA or RNA

• And a protein coat - capsid

• Some are enclosed by an envelope

• Some viruses have spikes

• Infect only specific cells in a specific

host

• Obligate parasites – need living cells

History of Virology

• 1892 – viruses 1st mentioned

– Russian Bacteriologist – Dimitri Iwanowski

– TMD – tobacco mosaic disease • Filtered plant sap

• Liquid still infectious

• 1935 – TMV isolated and purified

– American Chemist – Wendell Stanley

• Virus = Latin for poison

Virus Sizes

Figure 13.1

To Study Needed

to Develop:

EM (TEM & SEM)

Ultracentrifuge

& tissue culture

Most 20nm -200nm

Growing Viruses

• Viruses must be

grown in living cells.

– Bacteriophages form

plaques on a lawn of

bacteria.

Growing Viruses

• Viruses 1st

cultivated in live

animals

• or in embryonated

eggs

Figure 13.7

Growing Viruses • Next Tissue culture techniques were

developed.

– Continuous cell lines may be maintained

indefinitely.

Figure 13.8

How do you see growing virus in living cells?

Virus Identification

Figure 13.9

Cytopathic effects - CPE

Figure 13.3a

Cytopathic Effects

TMV

Categorizing Viruses

1. Genome compostion – RNA or DNA • dsDNA

• ssRNA

• ssDNA

• dsRNA

Categorizing Viruses

2. Capsid – made up of capsomeres

(proteins) differentiated by symmetry

Helical viruses

• capsomeres arranged in a tube, nucleic

acid inside

Polyhedral viruses

• Icosahedral (20 faces, 12 corners)

Complex viruses

• capsid + tail + tail fibers + pins + face plate

Helical Viruses

Figure 13.4a, b

Polyhedral Viruses

Figure 13.2a, b

Complex

Viruses

Figure 13.5a

Categorizing Viruses

3. Presence of Envelope

• Is the capsid surrounded by a phospholipid bilayer

derived from the hosts plasma membrane

Viruses and Host Range

• Viruses have specific hosts

– Plant

– Animal

– Bacteria

• Host Range

– Determined by specific receptors on the host

cell surface

– What are these used for normally?

Viral Taxonomy • Viruses named after

– the disease they cause

• Poliovirus, mumps virus, measles virus

– the river near place of isolation

• Ebola virus

• Viruses that infect bacteria

– Are called bacteriophages

– Letters and numbers are used for names

• T4, λ phage

How are bacteria named?

Viral Life Cycles

• Bacteriophage

– Lytic cycle

– Lysogenic cycle

• Viruses

– DNA virus

– RNA virus

– Retrovirus

• Attachment Attachment of virus to bacterial cell

• Entry Entry of nucleic acid into host cell

• Synthesis Production of nucleic acid & proteins

• Assembly Nucleic acid and capsid proteins

assemble

• Release Viruses burst out of the host cell

Multiplication of Viruses in a

Bacterial Host Cell

Figure 13.8

Lytic Replication of Bacteriophages

Figure 13.8

Lytic Replication of Bacteriophages

Figure 13.9

Lytic Phage Replication Cycle

• Lytic cycle Phage causes lysis and

death of host cell

• Lysogenic cycle Prophage DNA

incorporated in host DNA (Temperate phages)

The Lysogenic Cycle

Figure 13.12

Specialized Transduction

Figure 13.13

Prophage exists in galactose-using host (containing the gal gene).

Phage genome excises, carrying with it the adjacent gal gene from the host.

Phage matures and cell lyses, releasing phage carrying gal gene.

1

2

3

Prophage

gal gene

gal gene Bacterial DNA

Galactose-positive donor cell gal gene

Phage infects a cell that cannot utilize galactose (lacking gal gene).

4

Galactose-negative recipient cell

Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA.

5

Lysogenic cell can now metabolize galactose.

6

Galactose-positive recombinant cell

Viral Life Cycles

• Bacteriophage

– Lytic cycle

– Lysogenic cycle

• Viruses

– DNA virus

– RNA virus

– Retrovirus

• Adsorption Virus attaches to cell membrane

• Penetration By endocytosis or fusion

• Uncoating By viral or host enzymes

• Replication Production of nucleic acid & proteins

• Assembly Nucleic acid and capsid proteins

assemble

• Release By budding (enveloped viruses) or

rupture

Multiplication of Viruses in an

Animal Host Cell

• Same basic replication pathway as

bacteriophages

• Differences result from

–Presence of envelope around some viruses

–Eukaryotic nature of animal cells

–Lack of cell wall in animal cells

Replication of Animal Viruses

• Chemical attraction

• Animal viruses do not have tails or tail

fibers

• Have glycoprotein spikes or other

attachment molecules that mediate

attachment

Adsorption of Animal Viruses

Figure 13.3b

Viral Spikes

Figure 13.12ab

Penetration and Uncoating of Animal Viruses

Figure 13.12c

Penetration and Uncoating of Animal Viruses

Adsorption, Penetration, and

Uncoating

Figure 13.14

• Each type of animal virus requires different

strategy depending on its nucleic acid

• Must consider

–How mRNA is synthesized?

–What serves as template for nucleic acid

replication?

Replication of Animal Viruses

Replication & Assembly of DNA Virus

Figure 13.15

Virion attaches to host cell

Virion penetrates cell and its DNA is uncoated

Early transcription and translation; enzymes are synthesized

1

2

3

DNA

Late transcription; DNA is replicated

4

Late translation; capsid proteins are synthesized

5

Virions mature 6

Capsid

Papovavirus

Host cell

DNA

Cytoplasm

Virions are released 7

Capsid proteins

mRNA

Replication & Assembly in

RNA Viruses

Figure 13.17

Replication & Assembly of a Retrovirus

Figure 13.19

Retrovirus penetrates host cell.

Virion penetrates cell and its DNA is uncoated

The new viral DNA is tranported into the host cell’s nucleus and integrated as a provirus. The provirus may divide indefinitely with the host cell DNA.

1

2

3

Envelop

Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that codes for the retrovirus capsid and envelope proteins.

4

Mature retrovirus leaves host cell, acquiring an envelope as it buds out.

5

Capsid Reverse transcriptase

Virus Two identical + stands of RNA

DNA of one of the host cell’s chromosomes

Provirus

Host cell

Reverse transcriptase

Viral RNA

RNA

Viral proteins

Identical strands of RNA

Table 13.3

Summary of Replication &

Assembly of Animal Viruses

Release of an enveloped virus by

budding

Figure 13.20

Figure 13.13

Release of Enveloped Viruses

by Budding

• DNA viruses - assembly in nucleus

• RNA viruses develop solely in cytoplasm

• Number of viruses produced &released depends on

– type of virus

– Size of virus

– initial health of host cell

• Enveloped viruses cause persistent infections

• Naked viruses released by exocytosis or may

cause lysis and death of host cell

Assembly and Release of Animal Viruses

• Some viruses can remain dormant

• May exist for years with no viral activity,

signs, or symptoms

• Some latent viruses do not become

incorporated into host chromosome

• When provirus is incorporated into host DNA,

condition is permanent; becomes permanent

physical part of host’s chromosome

Latency of Animal Viruses

Summary of Bacteriophage and

Animal Virus Replication

Table 13.4

Cytopathic Effects

Figure 13.9

Cytopathic effects - CPE

Causes of Cytopathic Effects

1. Host cell DNA, RNA, and protein synthesis has been stopped by the virus

2. Fusion of the plasma membranes of many cells (Herpes viruses)

3. Inclusion bodies in host cytoplasm – Rabies = Negri bodies (viral particles)

4. Toxic effect of capsid proteins – Mumps virus & Influenza virus

5. Host Chromosomal Disruptions – Herpes virus

6. Transformation of cells into malignant cells

• Transformed/malignant cells = uncontrolled

cell division

• Oncovirus – has oncogenic effect on the

host (causes cancer)

– Genetic material of oncogenic viruses becomes

integrated into the host cell's DNA.

– Activated oncogenes transform normal cells into

cancerous cells.

– Transformed cells have increased growth, loss

of contact inhibition, and T antigens.

Cancer

Figure 13.15

Oncogene

Theory

• Ultraviolet light

• Radiation

• Carcinogens

• Viruses

Factors Involved in Activation of

Oncogenes

• Some carry copies of oncogenes as part of their

genomes

• Some stimulate oncogenes already present in host

• Some interfere with tumor repression when they

insert into host’s repressor gene

• Several DNA and RNA viruses are known to cause

~15% of human cancers

– Burkitt’s lymphoma

– Hodgkin’s disease

– Kaposi’s sarcoma

– Cervical cancer

How Viruses Cause Cancer

• Oncogenic DNA Viruses

– Adenoviridae

• Adenovirus -

adenocarcinomas

– Herpesviridae

• EBV – Burkitt’s lymphoma

– Poxviridae

• Smallpox, cowpox – misc.

– Papovaviridae

• Human papilloma virus –

cervical cancer

– Hepadnaviridae

• HBV – liver cancer

Oncogenic Viruses

• Oncogenic RNA viruses

– Retroviridae

• HTLV 1 (human T-

lymphotrophic virus)

• HTLV 2

– Causes leukemia in

adults

• Acute Viral Infections –

– Rapid onset, short duration • Influenza

• Latent Viral Infections

– Virus remains dormant and is later induced into activity

• HSV1 - Cold sores, varicella-zoster virus - shingles

• Chronic/Persistent Viral Infections

– Long duration, generally fatal • HCV, Subacute sclerosing panencephalitis (measles

virus)

Types of Infections

• Cytopathic effects

• Serological tests

– Detect antibodies against viruses in a patient

– Use antibodies to identify viruses in

neutralization tests, viral hemagglutination, and

Western blot

• Nucleic acids

– RFLPs

– PCR

Virus Identification

• 1982 purposed by American neurobiologist

– Stanley Prusiner

• Infectious proteins

• Transmissible by ingestion, transplant, & surgical

instruments

• Spongiform encephalopathies:

– Sheep scrapie, Creutzfeldt-Jakob disease, Kuru, fatal

familial insomnia, mad cow disease (bovine spongiform

encephalopathy)

– Large vacuoles in the brain

Prions

Prions

• Prion = proteinaceous infectious particle

– Reduced infectivity = proteases, not radiation

• PrPC, normal cellular prion protein

(glycoprotein), on cell surface

• PrPSc, scrapie protein, accumulate in brain

cells forming plaques

• Two stable tertiary structures of PrP

–Normal functional structure with α-helices called

cellular PrP

–Disease-causing form with β-sheets called prion

PrP

• Prion PrP converts cellular PrP into prion PrP

by inducing conformational change

Characteristics of Prions

Figure 13.21

Tertiary Structures of PrP

• Normally, nearby proteins and

polysaccharides force PrP into cellular shape

• Mutations in PrP gene result in initial

formation of prion PrP

• When prions present, they cause newly

synthesized cellular PrP to refold into prion

PrP

Characteristics of Prions

Prions

Figure 13.21

PrPc

PrPSc

1 2 3 4

5 6 7 8

Endosome

Lysosome

• All = fatal neurological degeneration,

– deposition of fibrils in brain,

– & loss of brain matter

• Large vacuoles form in brain

– = spongy appearance

• Spongiform encephalopathies – BSE, CJD,

kuru

• Only destroyed by incineration; not cooking

or sterilization

Prion Diseases

Figure 13.22

Scrapie in Sheep

Kuru